There are several strategies you can use to limit the effectiveness of a denial of service (DoS) attack when using sendmail. Here are a few approaches you can consider:
Use rate limiting: You can configure sendmail to limit the number of messages that can be sent by a single sender within a given time period. This can help prevent a single sender from overwhelming the server with a large number of messages.
Use connection limits: You can also configure sendmail to limit the number of connections that can be made to the server by a single sender. This can help prevent a single sender from using up all of the available connections to the server.
Use a load balancer: If you have multiple sendmail servers, you can use a load balancer to distribute incoming traffic among the servers. This can help prevent a single server from being overwhelmed by a large number of connections.
Use a firewall: You can use a firewall to block traffic from known malicious IP addresses or to rate limit incoming connections.
Use a content filter: You can use a content filter, such as SpamAssassin, to block messages that are likely to be spam or that contain malicious content. This can help reduce the overall load on the server and make it more difficult for attackers to use the server to send spam or other malicious messages.
It is important to note that no single strategy will completely prevent a DoS attack, and you may need to use a combination of these approaches to effectively protect your server.